Crossword Genius

Troops look cross mounting guard at entrance (10)

Ross

I believe the answer is:

doorkeeper

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'guard at entrance' is the definition.
(guard by a door)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for doorkeeper that I've seen before include "He attends an opening" , "Admission controller" , "Person on duty at entrance" , "Porter" , "One on duty at building's entrance" .)

I've seen this clue in The Guardian.
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